Hi everyone, I’m a bit confused about the currents in my FOC-controlled PMSM AC motor.

At maximum speed and with a load connected, I measured the following currents using a current probe:

• Idc,bus = 1,3 A (current on 48V DC bus)
• Iphase,rms = 2 A (rms current on the phase)
• Iphase,span = [-3,7 .. 3,9] A (rms span/range current on the phase)
• Iq,span = [1 .. 3,9] A (quadrature current, I measured this value in the firmware)

By the term "span" I mean the maximum and minimum values (in square brackets), so roughly the range.

The current in quadrature is not "constant" because my motor has two rollers that press alternately against the load... so we can say that the current (and therefore the torque) is variable, but that shouldn't be the point of my query.

1) If I set Iq,max = 20A via firmware .. will Iphase,max then be 20A? Or are these two independent values?

From my measurements, it seems the answer is "yes, the maximum values are the same" … but I’d like to hear the opinion of someone who knows more about the subject. To confirm this, in the event of a motor stall, one of the three phases has reached exactly the Iq,max value (the others have stopped earlier at different values, as is to be expected)

2) So, if it is true that Iq,max = Iphase,max … does this hold for all other values? Such as the average, the RMS and the minimum values? … for example, for the minimum value, the equality no longer holds because Iq cannot be less than 0, whereas (as you can see from my measurements) Iphase,min is negativ

 

Not all of us are conversant with all of the abbreviated terms that you have used in your inquiry.
But it seems that you are asking if the currents in a split phase motor are equal. In theory they should be the same, or at least fairly close.
There is a similar class of motors that are reversable, with one phase powered directly and the second, identical winding, fed thru a series capacitor. Those motors are reversable by simply changing which winding is directly powered.

 

Not all of us are conversant with all of the abbreviated terms that you have used in your inquiry.
But it seems that you are asking if the currents in a split phase motor are equal. In theory they should be the same, or at least fairly close.
There is a similar class of motors that are reversable, with one phase powered directly and the second, identical winding, fed thru a series capacitor. Those motors are reversable by simply changing which winding is directly powered.

Hi, thanks for your reply.

I’ve added a few more details (in brackets).
I’m asking if, in Field-Oriented Control and PMSM, the quadrature current is the same as the RMS current measured on the phases

 

I think that for the torque to be consistent the currents will need to be the same.

 

Not all of us are conversant with all of the abbreviated terms that you have used in your inquiry.
But it seems that you are asking if the currents in a split phase motor are equal. In theory they should be the same, or at least fairly close.
There is a similar class of motors that are reversable, with one phase powered directly and the second, identical winding, fed thru a series capacitor. Those motors are reversable by simply changing which winding is directly powered.

In a 1ph / split phase cap start motor, under 1/2 HP are likely to posses identical windings, these are capable of using the series cap with either winding , over this size of motor , the start winding is invariably quite higher resistance than the main run winding, and the only one to make use of the start cap.

 

Iq and phase current are related, but they are not the same quantity in all situations.

Iq is the torque-producing current after Clarke/Park transformation, so it is expressed in the rotating dq reference frame. The phase currents are the actual sinusoidal currents flowing in the motor phases. Because of that, you should not directly compare Iq average/min/max with one phase current average/min/max as if they were the same signal.

If Id is close to zero, then the magnitude of the stator current vector is mainly Iq. In that case, limiting Iq to 20A limits the torque-producing current vector, but an individual phase current will still be a sinusoidal quantity and can be positive or negative depending on electrical angle.

So for your questions:

1. Iq,max = 20A does not literally mean every phase current is 20A all the time. It means the commanded q-axis current is limited to 20A. Depending on scaling convention, the phase peak current may be similar to Iq, but the RMS phase current will not be the same value.
2. The equality does not hold for average, RMS, or minimum values. Iq can remain positive because it represents torque direction, while an individual phase current naturally swings positive and negative.

The DC bus current is also different again. It represents input power from the DC link, so it depends on motor speed, torque, inverter losses, and efficiency. At low speed or stall, you can have high phase/Iq current while DC bus current is relatively low because mechanical power is low.

So I would treat Iq as the controlled torque-current component, and phase current as the physical winding current waveform. They are connected mathematically, but they are not interchangeable measurements.

 

Iq and phase current are related, but they are not the same quantity in all situations.

Iq is the torque-producing current after Clarke/Park transformation, so it is expressed in the rotating dq reference frame. The phase currents are the actual sinusoidal currents flowing in the motor phases. Because of that, you should not directly compare Iq average/min/max with one phase current average/min/max as if they were the same signal.

If Id is close to zero, then the magnitude of the stator current vector is mainly Iq. In that case, limiting Iq to 20A limits the torque-producing current vector, but an individual phase current will still be a sinusoidal quantity and can be positive or negative depending on electrical angle.

So for your questions:

1. Iq,max = 20A does not literally mean every phase current is 20A all the time. It means the commanded q-axis current is limited to 20A. Depending on scaling convention, the phase peak current may be similar to Iq, but the RMS phase current will not be the same value.
2. The equality does not hold for average, RMS, or minimum values. Iq can remain positive because it represents torque direction, while an individual phase current naturally swings positive and negative.

The DC bus current is also different again. It represents input power from the DC link, so it depends on motor speed, torque, inverter losses, and efficiency. At low speed or stall, you can have high phase/Iq current while DC bus current is relatively low because mechanical power is low.

So I would treat Iq as the controlled torque-current component, and phase current as the physical winding current waveform. They are connected mathematically, but they are not interchangeable measurements.

Hi, thank you for your reply

In terms of design, is it correct to calculate Iq (in the FOC method) "simply" by reversing the torque formula?
In other words, given the number of poles of the motor, the flux and Tmax, I can find Iq,max

Or is there anything else I need to take into account?

 

Hi, thank you for your reply


In terms of design, is it correct to calculate Iq (in the FOC method) "simply" by reversing the torque formula?
In other words, given the number of poles of the motor, the flux and Tmax, I can find Iq,max

Or is there anything else I need to take into account?

You're welcome.

As a first approximation, yes. If you're using a PMSM with FOC and operating with Id = 0, then the torque equation can be rearranged to estimate the required Iq for a desired torque.

However, in a real design you should also consider: (I've mentioned a few)

• Current limits of the inverter and motor windings
• Thermal limits (continuous vs peak torque)
• Flux weakening operation at higher speeds
• Motor parameter accuracy (flux linkage values are not always exact)
• Safety margin for transient loads

So the torque equation is usually a good starting point for estimating Iq,max, but the final limit is often determined by the thermal and electrical constraints of the overall system rather than the torque equation alone.